1. Field of the Invention
The present invention relates generally to the field of electro-optical inspection systems, and more particularly to grey scale wafer inspection to detect defects on processed semiconductor wafers and the like.
2. Description of the Related Art
Integrated circuits are produced using photolithographic processes on specimens, such as silicon wafers. These processes can employ photomasks or reticles and a light source to project a circuit image onto the silicon wafer. The presence of surface defects on the wafer, particularly after undergoing the photolithographic process, is highly undesirable and adversely affect the resulting circuits. Defects can result from, but not limited to, a portion of the pattern being absent from an area where it is intended to be present, a portion of the pattern being present in an area where it is not intended to be, chemical stains or residues from the photomask manufacturing processes which cause an unintended localized modification of the light transmission property of the photomask, particulate contaminates such as dust, resist flakes, skin flakes, erosion of the photolithographic pattern due to electrostatic discharge, artifacts in the photomask substrate such as pits, scratches, and striations, and localized light transmission errors in the substrate or pattern layer. Since it is inevitable that defects will occur, these defects are preferably located and repaired prior to use.
Methods and apparatus for detecting defects have been generally available. For example, inspection systems and methods utilizing laser light are available to scan the surface of substrates such as photomasks, reticles and wafers. These laser inspection systems and methods generally include a laser source for emitting a laser beam, optics for focusing the laser beam to a scanning spot on the surface of the substrate, a stage for providing translational travel, collection optics for collecting either transmitted and/or reflected light, detectors for detecting either the transmitted and/or reflected light, sampling the signals at precise intervals and using this information to construct a virtual image of the substrate being inspected.
Although such systems work well under many conditions, ongoing work in the area seeks to improve existing designs to enable higher degrees of sensitivity, increase the ability to classify and quantify defects, and to allow faster scanning speeds and higher throughput. As the complexity of integrated circuits has increased, the demands on the inspection of the integrated circuits have also increased. Both the need for resolving smaller defects and for inspecting larger areas have resulted in greater magnification requirements and greater speed requirements.
During the manufacture of wafers having patterns etched thereon, automated inspection of the wafer is performed to ensure freedom from the aforementioned defects. Various methods for the inspection of patterned masks, reticles, or the wafer surface are currently available. Many of the available processes, such as ion implant, oxidation, CVD (chemical vapor deposition), etching, and so forth cause a difference in the optical appearance of the wafer before and after such a process occurs. Changes in the appearance of a wafer before and after a process step can be optically detected by measuring the composite greyscale difference of the whole wafer at preprocessing time and subsequently at postprocessing time. This method can be utilized to verify complete or partial completion of a process. Previous systems have used single pixel grey scale difference comparisons between preprocessed and postprocessed wafers to determine the defects on the wafer and the quality of the process employed. These systems generally made a pixel by pixel comparison between the preprocessed and postprocessed surface representations, and generally required a calibration or matching process before making such a comparison. Single pixel greyscale difference measurements before and after processing are generally not sensitive enough to detect the difference of the optical appearance change.
Other previous systems used reflectivity measurements to determine wafer defects and the quality of the process. Use of reflectivity measurements can result in problems due to process variation induced noise. Gross defects can be detected on wafers using reflectivity measurements, but inspection sensitivity and ultimate results can be compromised by intrinsic process variation. In this instance, process variation may be interpreted as noise by the system, and requires desensitizing the inspection to prevent false positives, or false indications that a defect exists. Use of reflectivity measurements places entire classes of defects below the detection threshold of the inspection system, and thus the slight change in greyscale value can go undetected.
Based on the foregoing, it would be beneficial to provide a system which did not include certain drawbacks associated with previous wafer inspection systems.